Effect of sludge solids properties on sludge dewatering operations

The purpose of this research was to investigate the influence of some pertinent sludge properties on dewatering and the effect of conditioning chemicals on those sludge solids properties and optimum conditioning dosage for various dewatering operations as determined by the mixing intensities.;Unconditioned Waste Activated Sludge (WAS) from a conventional wastewater treatment facility was used in this investigation. The sludge solids properties, colloidal charge, particle size distribution and total carbohydrate concentration (polymer content) were studied with regards to their effect on dewatering as determined by the specific resistance to filtration. The Chemineer mixing apparatus was used to generate high mixing intensities, hence simulate various high intensity dewatering operations. Ferric Chloride and Cationic Polymer were the most effective conditioning agents with respect to this sludge. Shearing of unconditioned sludge increased the proportion of particles in the size range of 0.45-2.5 microns and increased the total carbohydrate content. Both these effects made the sludge more difficult to dewater. Accordingly, when the total carbohydrate content was enhanced by acclimating sludge at a high carbon to nitrogen ratio, resistance to filtration was increased.;Sludge conditioned with cationic polymer, decreased the specific resistance, increased filtration rate and the filter cake solids concentration. The charge and pH on the sludge solids after conditioning had little or no influence on the specific resistance, possibly indicating that particle bridging might be the predominant mechanism.;The mass fraction of particles in the size range of 0.45-2.5 microns and total carbohydrate content decreased as the sludge dewaterability was enhanced. This observation supported the hypothesis that biocolloids have a slime coat and reduction of the biocolloids, hence the slime coat may enhance dewaterability.;Similar conclusions were drawn with ferric chloride as the conditioning agent with some exceptions, pH and colloid charge decreases as the specific resistance decreases, charge is driven towards electroneutrality, suggesting the possibility of charge neutralization as the predominant mechanism.;Optimum dosage of conditioning chemicals was established and was found to be higher at higher mixing intensities. Thus, using conventional jar testing for determining the optimal dose would underrate the actual requirement.